Active Learning Strategy - ABG Tic Tac Toe - Page 2

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1. Lots of great questions! I love that you are digging into the material to get a good understand. I'll try to answer all your questions, but let me know if I miss one

7.4 pH: would be impossible to identify the imbalance if this was all you knew. Therefore, for purposes of testing, you would never see this as a test question unless the other values indicated a normal ABG. That being said, if you saw this in a real patient, then you would just have to look at the patient as a whole, what are their sx, dx? That information can help you identify the type of imbalance. For example, if the pt was vomiting profusely, then they are losing a lot of (stomach) acid and probably are in some form of compensated metabolic aklylosis. So the pt sx can add to the interpretation.

Base excess: Don't necessarily need this value, because it pretty much tells you the same thing as the HCO3, at least in the problems you encounter as a nursing student. I have never used it, but I would guess it might have more value in a critical care specialty.

pH 7.45 (7.35-7.45)
HCO3 21 (22-26)
PaCO2 29 (35-45)
I'm with you on how I would interpret this problem. I would hazard to say that the other rationale might be incorrect (or more advanced than a basic, beginner nurse would need to know). This is how I would do this problem. I always write out the normal values next to the ABGs so I don't confuse myself The pH is normal, but high (more basic). Glancing at the other values, I quickly see that they are both abnormal. Therefore I know that the pH, although in the normal range, is actually "fully-compensated." Since the 'normal' pH is on the high-side (more basic), I know that the primary underlying problem is going to be "fully-compensated _________ alkylosis." Now to fill in the last blank. Look at the HCO3, and it is low, which means less base, or "metabolic acidosis." Then PaCO2 is also low, but this means less acid, or "respiratory alkylosis."

So in order to fill-in-the-blank for the primary mechanism, now we go back to the pH. Which of the 2 imbalances fit the pH imbalance? In this case, the pH is "more basic," so the primary mechanism is "respiratory alkylosis." That would make the secondary mechanism, by default, "metabolic acidosis."

So in short, I would use the rule of "which imbalance is pH closer to?" And that has yet to steer me wrong! Where was this ABG problem at, on the practice website that you have been using?

It's great to understand ABGs at the level it seems like you're getting to! I did the same thing when I was in nursing school (and I still remember most of it, so I think it's a good way to do it!). But don't stress out! Remember, NCLEX wants to make sure that you will be a basic, safe new nurse. That means you need to be able to interpret basic ABGs. The most complex interpretation you'll probably see is partially- or fully-compensated imbalance, and they will make it obvious for you (if you understand how to interpret the ABG values, that is). Then if you go into a specialty where you will encounter more complex interpretations, you will have the opportunity to learn more then. But the examples that you are posting are pretty advanced.

Let me know if you have more questions!!
Last edit by RNTutor on Jun 5, '10
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2. This ABG Tic-Tac-Toe Video helped me:

3. Thank you so much for posting this. I have a terrible time with ABG's! I will be taking my NCLEX next month and this will help greatly with studying!
4. I taught ABG interpretation for yrs in a way that made it pretty foolproof. You will make your own key to interpret ABG's, and will be able to reproduce it from memory any time you need to with very little trouble if you learn a very few **key concepts**, labeled **thus**..

Take a piece of paper. Make a big box on it, then draw vertical and horizontal lines on it so you have four boxes. I will try to make this come out, but...you should have

AB
CD

where the four boxes a,b,c,d are such that a is above c and b is above d. You don't need to label the boxes a,b,c,d, just get them in the right alignment. (This is WAY easier with a whiteboard, bear with me).

*Inside* each of the 4 boxes write the following, down the left edge:
pH
CO2
Bic

Now, OUTSIDE the big box do the following: above the "A" box write "resp"; above the "B" box write "metabolic"

To the left of the "A" box write "acidosis" and to the left of the "C" box write "alkalosis"

Now you have a "resp" column and a "metabolic" column, an "acidosis" row and an "alkalosis" row. So you have respiratory acidosis and alkalosis boxes, metabolic acidosis and alkalosis boxes.

With me so far?

Now, you're going to label the PRIMARY DERANGEMENTS, so later you can tell what's the derangement and what's the compensation. OK? In the respiratory column, underline CO2's. In the metabolic column, underline the Bicarb's. That's because in **respiratory disorders, the CO2 gets messed up**, and in **metabolic disorders, the Bicarb is messed up**. You knew that, or could figure it out pretty quick if you thought about it, right? Thought so.

Now. You are going to put upward-pointing and downward-pointing arrows next to the pH, CO2, and Bicarb labels inside every box. Ready?

pH first. In the "alkalosis" row, make up arrows next to pH, because **pH is elevated in alkalosis (by definition)**. Put down arrows in the acidosis row's pHs, because **acidosis means a lower that nl pH**.

Remember that **CO2 is (for purposes of this discussion and general clinical use) ACID** and **Bicarb is ALKALINE** (this is the end of the key concepts. Not too bad, huh?). (oops, I forgot: **nls are generally accepted as pH 7.35-7.45, CO2 35-45 (nice symmetry there), bic 19-26**)

Now go to the box that is in the respiratory column and the acidosis row. Figured out that CO2 must be elevated? Good. Put an up arrow next to that CO2. Go to the respiratory alkalosis box. Figures that CO2 must be low to cause this, right? Put a down arrow next to that CO2.

OK, now go to the next column, the metabolic one. I think you can figure out what happens here: in the metabolic alkalosis box, put an up arrow next to the Bic, because high bicarb makes for metabolic alkalosis. Put a down arrow next to the Bic in the metabolic acidosis box, because in metabolic acidosis the bicarb is consumed by the acids (like, oh, ASA) and is low.

You are now going to put arrows next to the blank spots in your boxes that show compensatory movements. Ready? OK, what does your body want to do if it has too much acid? Right, retain base. Yes, of course if your body has too much acid it would like to get rid of it...but if it can't do that, then retaining bicarb is the compensation. So for every elevated CO2 you see, put an up arrow with its bicarb.( Chronic CO2 retainers always have elevated bicarbs, and this is why.) You will find an up arrow next to the CO2 in the resp/acidosis box.

So if your body is short on acids, what does it do? Right, excrete base. So put a down arrow next to the bicarb in the resp/alkalosis box, because chronic low CO2 makes the body want to get back into balance by getting rid of bicarb. However, remember that it takes a day or two for the kidney to do this job, and if you have nonfunctioning kidneys they won't do it at all.

Likewise in the metabolic/alkalosis box, a high bicarb makes your body want to retain acid, increasing CO2 being the fastest way to do that because all you have to do is hypoventilate, to bring your pH back towards nl. Put an up arrow next to the CO2 in the met/alk box. See the pattern here? Put a down arrow next to the CO2 in the met/acidosis box, because if your body has too much acid in it (think : ASA overdose? DKA?) it will want to get rid of CO2 to compensate, and the fastest way to do that is to hyperventilate. This is why patients in metabolic acidosis are doing that deep, rapid breathing thing (Kussmaul's respirations).

OK, I hear you wailing: but how do I know whether that elevated or decreased CO2 or Bicarb in my ABG report is primary or compensatory?

Well, now you have your key. So take your ABG reports and look at them. Say, try these. (Notice that O2 levels have nothing to do with acid-base balance ABG interpretation) (OK, if you are VERY hypoxic you can get acidotic...but you see that in the metabolic component, not the O2 measurement, because it's lactic ACID your body is making if it's working in an anaerobic way)

1) pH = 7.20, CO2 = 60, Bic = 40.

2) pH = 7.54, CO2 = 60, Bic = 40
pH here? This is alkalosis, with a high pH.
The only box where pH is high and CO2 & Bic are both elevated is metabolic alkalosis with respiratory compensation. Sometimes you'll see this in people who have a bigtime antacid habit. Really. (You can get a short-term metabolic alkalosis with rapid severe vomiting, because the body's nl balance between acid and base has been disrupted due to a sudden loss of acid. Things will equilibrate pretty quickly, though, all things considered.)

So even though you have identical abnormal CO2's and Bicarbs, you can look in your boxes, find the match, and see what you have. Remember you underlined the primary disorder in each box?

3) pH = 7.19, CO2 = 24, Bic = 12. Bingo, you found it: an acidosis where the CO2 and the Bic are both abnormally low. Only fits in the metabolic acidosis box, so you have a metabolic acidosis with a respiratory compensation effort. Incidentally, this is what you see in diabetic ketoACIDOSIS, when they come in huffing and puffing to blow out that CO2 because their ketosis is so high. Also you see this picture in ASA OD's, because this is acetylsalicylic ACID they ate, and the fastest way to get rid of acid is to blow it off via hyperventilation. Increasing your bicarb takes 24-48 hrs. Another quick way to get a metabolic acidosis is to poop out a lot of diarrhea, because you lose a lot of bicarb that way. Another classic place for this is in mesenteric artery thrombosis, in which you have a lot of ischemic bowel sitting in there screaming for oxygen and making lactic acid when it can't have any.

Trust me, you'll never go wrong with it, and you can recreate it anytime. It doesn't really even matter how you set up your boxes, so long as you have a metabolic/respiratory axis and an acid/alkaline axis. Rotate your paper and you'll see what I mean.

Why don't I care about PaO2 here? Well, because ABG's mostly tell you about A/B balance and CO2 and Bicarb, that's why. Probs with them can be serious probs without any abnormality in oxygenation at all.

Remember that PaO2 (arterial oxygen, measured in torr or mmHg) is not the same as SpO2,( hemoglobin saturation, a percentage of red cells carrying oxygen). if you think they are, your pt could be in serious trouble before you do anything. There is a nomogram that shows you the relationship between arterial oxygen and saturation, which I regret I cannot reproduce here. But you can sketch out a basic version...

Draw a graph where sats are on the vertical (left) axis and PaO2's are on the horizontal (bottom) axis. Draw little shaded band across the top at the 95%-100% sat areas. That's your normal saturation. Draw a few dots there indicating a line of PaO2's of 80-100, because those are normal PaO2's.

Now draw a dot for SpO2 of 90 and PaO2 of about 75. Now, another dot showing SpO2 of 85 and PaO2 of about 60. Another dot: SpO2 of about 80 and PaO2 of about 55. Connecting all these dots should give you a sort of S curve, indicating that while the top is pretty flat in the PaO2 80-100, SpO2 95-100 range, PaO2 drops off like a shot at decreasing SpO2 levels.

Your pt with a sat of 85 is not doing OK, he's in big trouble. While a PaO2 of 75 torr isn't too bad at all, a SAT of 75% is heading for the undertaker unless dealt with.

Here's my very favorite ABG of all time: pH = 7.11, PaO2 = 136, PaCO2 = 96, bicarb = 36.

What happened to this lady? What will happen next?